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Polymorph

Polymorph

A polymorph is a chemical composition that can crystallize into more than one type of structure. This results in different minerals with identical compositions and distinguished by their crystallography.

Some common examples of polymorphs are calcite and aragonite. The composition of these two minerals is CaCO3, but calcite is rhombohedral while aragonite is orthorhombic. Diamond and graphite , both of which are pure carbon , are also polymorphs. Diamond, however, is cubic while graphite is hexagonal. Pyrite is the cubic form of FeS2, marcasite, the orthorhombic version.

A single chemical composition that can form polymorphs does so as a response to varying conditions of formation. The temperature , pressure, and chemical environment all affect the crystallization process and can determine the resulting polymorph. For example, diamond requires very high pressure to crystallize, while graphite forms at lower pressures. For the composition CaCO3, calcite is the high temperature-low pressure polymorph while aragonite forms at higher pressures and lower temperatures.

Many polymorphs are only stable within a certain range of conditions and solid-state transitions from one polymorph to another are possible. When low-quartz, which is rhombohedral, is heated to above 1063°F (573°C), it instantaneously goes through an internal structural displacement, or shift, to form hexagonal high-quartz. This type of polymorphic transition is reversible if the temperature is lowered. Other polymorphic transitions involve extensive internal rearrangement and reconstruction of the crystal and subsequently require significantly more energy. The examples of diamond-graphite, pyrite-marcasite, and calcite-aragonite are all known as reconstructive transitions. The large amounts of energy required to effect these polymorphic changes makes the resulting mineral more stable and the process less reversible than with a displacive transition.

See also Crystals and crystallography; Mineralogy

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polymorphism

polymorphism The existence of two or more distinctly different forms (morphs) within a plant or animal species. An example is the caste system of social insects, in which there are workers, drones, and queens. This is an environmental polymorphism (see polyphenism), i.e. the differences are caused by environmental rather than genetic factors, in this case by the larvae receiving different types of food. There are also heritable or genetic polymorphisms, in which differences are the result of inherited characteristics. Two forms of genetic polymorphism exist: transient polymorphism, in which a particular form is in the process of spreading through a population, causing the relative proportion of phenotypes to alter; and balanced polymorphism, in which two or more forms coexist in a stable ratio within a population, each form possessing both advantageous and disadvantageous characteristics. An example of balanced polymorphism is the occurrence of sickle-cell disease, a genetic disease that principally affects Black populations of central Africa and is characterized by an abnormal form of the blood pigment haemoglobin (haemoglobin S) and sickle-shaped red blood cells. Three different types of individual occur in such populations: those who have two genes (AA) for normal haemoglobin and therefore do not suffer from the disease; those with one normal and one abnormal gene (AS), who are described as having the sickle-cell trait and generally suffer no symptoms; and those with two abnormal genes (SS), who suffer a chronic and eventually fatal form of anaemia. Normally such a harmful gene would have been eliminated from the population by the process of natural selection, but it is maintained in this case because people with the sickle-cell trait are resistant to a severe form of malaria endemic in central Africa. Compare mutation.

See also restriction fragment length polymorphism; single nucleotide polymorphism.

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polymorphism

polymorphism
1. In genetics, the existence of two or more forms that are genetically distinct from one another but contained within the same interbreeding population. The polymorphism may be transient or it may persist over many generations, when it is said to be balanced. Classical examples of polymorphisms are the presence or absence of banding in Cepaea snails, the number of spots on the wings of ladybirds, and eye colour in humans. All these are visible polymorphisms that can readily be seen in nature. Some polymorphisms, however, are cryptic and require biochemical techniques to identify phenotypic differences. Such techniques include gel electrophoresis of enzymes and other proteins, and the fragmentation of the DNA molecule by restriction enzymes (which allows the sequencing of nucleotides), both of which operate nearer to the level of the genotype.

2. In social insects, the presence of different castes within the same sex. See POLYTYPISM.

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polymorphism

polymorphism A feature of some modern high-level programming languages that allows arguments to procedures and functions to vary systematically over a whole class of data types, rather than being restricted to a single type. A simple example would be a function to find the length of a list. The code for such a function should be the same for lists of integers, lists of Booleans, or lists of anything. In a language like Pascal, however, the argument to such a function must have a single type; hence to handle both lists of integers and lists of Booleans, two functions would have to be defined. This can be avoided in languages (such as ML) that support polymorphic types like “list of alpha”, where alpha is a type variable standing for an arbitrary type. A polymorphic function is one that takes one or more arguments of polymorphic types.

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polymorphism

polymorphism
1. In genetics, the existence of two or more forms that are genetically distinct from one another but contained within the same interbreeding population. The polymorphism may be transient or it may persist over many generations, when it is said to be balanced. Classical examples of polymorphisms are the presence or absence of banding in Cepaea snails, the number of spots on the wings of ladybirds, and eye colour in humans. All these are visible polymorphism that can readily be seen in nature. Some polymorphism, however, is cryptic and requires biochemical techniques to identify phenotypic differences.

2. In social insects, the presence of different castes within the same sex. See also polytypism.

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polymorphism

polymorphism In genetics, the existence of 2 or more forms that are genetically distinct from one another but contained within the same interbreeding population. The polymorphism may be transient or it may persist over many generations, when it is said to be balanced. Some visible polymorphism can readily be seen in nature. Other examples are cryptic and require biochemical techniques to identify phenotypic differences. Such techniques include gel electrophoresis of enzymes and other proteins, and the fragmentation of the DNA molecule by restriction enzymes (which allows the sequencing of nucleotides), both of which operate nearer to the level of the genotype. See also POLYTOPISM.

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polymorphism

polymorphism Ability of elements or compounds to exist in more than one crystal form, with each having the same chemical composition but different physical properties due to differences in the arrangement of atoms. Examples are graphite and diamond (both C); alpha and beta quartz (both SiO2); and calcite (hexagonal) and aragonite (orthorhombic), both forms of CaCO3. The terms ‘dimorphism’ and ‘trimorphism’ describe (respectively) two and three different forms.

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polymorphism

polymorphism, of minerals, property of crystallizing in two or more distinct forms. Calcium carbonate is dimorphous (two forms), crystallizing as calcite or aragonite. Titanium dioxide is trimorphous; its three forms are brookite, anatase (or octahedrite), and rutile. Polymorphism of an element is called allotropy. The process was discovered (1821) by Eilhard Mitscherlich. See isomorphism; mineral; crystal.

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polymorphism

polymorphism The ability to crystallize in two or more different forms. For example, depending on the conditions under which it is solidified, the fat tristearin can form three kinds of crystals, each of which has a different melting point, namely, 54, 65, and 71 °C.

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polymorph

polymorph (polymorphonuclear leucocyte) (pol-i-morf) n. a type of white blood cell with a lobed nucleus and granular cytoplasm. See basophil, eosinophil, neutrophil.

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polymorph

polymorph One of several possible crystal forms of an element, compound, or mineral, all of which have the same chemical composition. See POLYMORPHISM.

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polymorph

polymorph See LEUCOCYTE.

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